1. Field of the Invention
The present invention relates to rotary valves, that is, valves in which a rotatable closure element is mounted in a flowway defined by the valve body so that it may be rotated between open and closed positions to control flow of fluids through the flowway.
2. Description of the Background
Rotary valves, such as butterfly or disk valves, ball valves, plug valves, globe valves and the like, are used in numerous industrial and military environments. In a typical rotary valve assembly, the valve body defines a fluid flowway and a cavity in which the valve element is disposed. There is also a suitable sealing assembly disposed in the valve cavity such that sealing between the valve body and the valve element can be effected to thereby prevent the flow of fluid through the valve when the valve is in the closed position. The sealing assembly used in many rotary valves is conventionally of an elastomeric and/or polymeric material although there are rotary valves which employ metallic seals and metal-to-metal contact to effect sealing. Indeed, valves which are in environments subject to fire or intense heat, which would virtually destroy a valve seat of an elastomeric or polymeric material, generally are provided with valve seals which are either composite in nature in the sense that there is a primary sealing section of a polymeric material and a secondary or backup sealing section of metal. Thus, in the event the polymeric sealing section is destroyed by fire, the secondary metal sealing section acts to effectively seal flow through the valve.
Although rotary valves employng only metallic seats do not suffer the disadvantages of having the seats destroyed when the valve is subjected to a fire or extremely intense heat, they suffer from certain infirmities. For one, fluid-tight sealing between a metal valve element and a metal seal is more difficult to achieve than sealing between a metal valve element and a resilient seal made of a polymeric material. Additionally, metallic seals are much more expensive to manufacture, present difficulties in installation and often times are subject to chemical attack by fluids flowing through the valve. Another problem with certain types of rotary valves employing metallic seals is that they present machining problems in that highly controlled tolerances must be maintained in order for the valve element and the valve seal to cooperate in effecting complete shutoff of flow through the valve.
Even in cases where metallic seals are employed in socalled fire-safe valves, it occasionally happens that the metallic seals, because they are constructed of very thin material, will distort under prolonged intense heat with the result that the valve will fail.